Automatic shut-off lighting controls are used to save electrical energy, and are often required by legislated energy codes. Today's control devices have multiple control outputs which are used to operate multiple lighting circuits in a particular area. The advantage of having multiple circuits feeding an area is that multiple levels of light can be achieved by selecting the number of circuits that are ON simultaneously. This multiple-level ability is also required by some energy codes so that lower levels of artificial illumination can be provided in an occupied area.
The type and arrangement of light fixtures is a factor in the application of multiple-circuit lighting controls. For example, alternate rows of lights can be fed from different circuits such that when only a portion of the lights are turned ON—same for OFF, the level of illumination is relatively even. This method can also be used with the light fixtures wired in a checker board pattern. Another common variation is the use of light fixtures with multiple ballasts, or specialty ballasts that can be fed from multiple circuits. This approach allows control of individual lamps within the fixture. For example, a light fixture with four lamps and two ballasts can provide illumination levels of 0%, 50%, or 100%. Another example is a light fixture with three lamps and two ballasts that can provide illumination levels of 0%, 33%, 66%, or 100%, achievable by having one ballast to provide energy to one lamp and the other to two lamps.
Multiple lighting levels can be controlled manually by a wall switch, automatically by a sensor, or both. An occupancy sensor can be used to automatically turn lights ON when a person enters an area and then turn lights OFF when all occupants have left an area. A light level sensor is often used in conjunction with this approach to prevent one or more light circuits from turning ON in response to occupancy. Natural light from windows, skylights or other sources adds to the illumination of the area. When a lighting control device determines that sufficient natural light reduces the need for artificial light, it will respond by allowing only a minimum level of artificial lighting to be automatically turned ON.
The multiple-circuit approach is also useful in situations where no natural light is available. An occupancy sensor will automatically turn ON lights when a person enters an area. To save energy, only a minimum level of light will be turned ON in response to this event. If a task in the area requires greater illumination, the occupant can manually turn ON additional light levels. The lighting control device will turn OFF all light circuits when the area is unoccupied. Only the minimum level will be restored on subsequent entries to the area.
Although these methods save energy, there are some detrimental side effects to the automatic shut-off control. If an occupancy sensor turns lights OFF long after the last occupant has left an area, energy is wasted. If an occupancy sensor turns lights ON and OFF too frequently, the lamp life is significantly diminished. The cost of replacing lamps, as well as the temporary loss of use for the area, is undesirable. A great deal of effort has been expended by the manufacturers of occupancy sensors to optimize timing.
Even if timing is optimized to solve the problem of frequent cycling, another problem remains. All such sensors respond to events by turning ON a designated primary circuit, with the designated secondary circuit turning ON only in response to the light sensor or a manual event. As a result, lamps connected to the primary circuit will cycle more frequently than lamps connected to the secondary circuit.
Replacement of lamps typically occurs by one of two methods: spot re-lamping, in which lamps are replaced when they fail, and group relamping, in which all lamps are replaced based upon a predictive maintenance calculation of lamp life. Lamp life is affected by the number of starts and total burn hours. In the spot re-lamping scenario, the lamps connected to the primary circuit will be replaced more often than lamps connected to the secondary circuit. The costs of spot re-lamping are very high since it requires multiple service events. In the group re-lamping scenario, the predictive maintenance calculation is normally based on the primary circuit, causing the secondary circuit lamps to be replaced while they still have much useful life. Since lamps often contain toxic materials, such as mercury, disposal of useful lamps is environmentally insensitive.